1. Field of the Invention
The present invention relates to a near field communication (NFC) antenna well suited for use in RFID (radio frequency identification, which is a generic name of contactless or proximity recognition techniques using electromagnetic waves) and a mobile device including an NFC communication portion and using the NFC antenna
2. Description of the Related Art
Recently, an RFID function is mounted in a standard manner on mobile devices, such as mobile phones. In past and existing techniques, a loop antenna has been used as an antenna of an NFC communication portion to realize the RFID function.
Recently, however, miniaturization or compactness and thinning are strongly demanded for mobile devices. In order to meet the demand, such a loop antenna, which is relatively large in size, cannot be mounted, such that it is difficult to secure a sufficiently necessary communication distance.
In addition, there have been developed ferrite antennas that are smaller in size than loop antennas, but have functional characteristics higher than or equivalent to the characteristics of loop antennas. Further, improvements have been made with respect to the communication distance by improving the reception sensitivity of an RFID LSI (large scale integration) circuit constituting an RFID control portion of the NFC communication portion.
The improvement, therefore, results in increasing the reception sensitivity, thereby making it possible to perform reception even in a remote site. At the same time, however, there occurs a problem in that interference is likely to receive from RFID readers/writers (reading/writing devices) other than a far-end communication device in operation. Particularly, a system employing ASK modulation (ASK: amplitude shift keying) likely to receive disturbance due to interference from other reading/writing devices.
As such, as reader/writer functions are increasingly mounted in various devices, and such devices increases in near sites, communication jamming due to interference increases. In this case, the various devices include, for example, office gate/door control devices (or, entrance/exit control devices), vending machines, green ticket reading machines in trains, notebook personal computers, and mobile phone terminals.
As countermeasures for such interference, it is advantageous to enhance the directivity of an NFC antenna, thereby to concentrate sensitivity on the direction of the far-end communication device, such as reading/writing device. As a method for enhancing the directivity, a method such as described herebelow is known.
FIGS. 5A to 5C are explanatory view showing means or manners for enhancing the directivity of a loop antenna. With reference to FIGS. 5A to 5C, numeral 1 represents the entirety of the loop antenna. As shown in FIG. 5A, the loop antenna 1 is formed in such a manner that a loop-shaped coil 3 is formed on one side of a flexible substrate 2 by printing, for example.
When current (current or induction current of a transmission signal) flows through the coil 3 of the loop antenna 1, as shown by a broken line 4 in FIG. 5B, a magnetic flux occurs spreading in exactly identical patterns to one another on the side having the coil 3 of the flexible substrate 2 and the reverse side. More specifically, the loop antenna 1 exhibits the same characteristics of directivity on the side having the antenna coil 3 of the flexible substrate 2 and the reverse side.
With reference to FIG. 5C, in order to enhance the directivity of the loop antenna 1 along the one direction, a magnetic material 5 having a high magnetic factor is adhered on any of the side, which has the antenna coil 3 of the flexible substrate 2, and the reverse side.
In the configuration thus formed, as shown by a broken line 6 in FIG. 5C, the magnetic flux on the side having the magnetic material 5 passes mainly through the inside of the magnetic material 5, but does not spread on the side having the magnetic material 5. Relatively, however, the magnetic flux on the side without the magnetic material 5 spreads to a long distance. Consequently, the reception sensitivity can be concentrated to the side without having the magnetic material 5, thereby making it possible to enhance the directivity in that direction.
With reference to FIGS. 6A to 6C, a manner for enhancing the directivity of a ferrite antenna will be described herebelow.
In FIG. 6A, numeral 11 represents the entirety of an example of a ferrite antenna. The ferrite antenna 11 of the example is of a bisectional type. As shown in FIG. 6A, the ferrite antenna 11 is configured such that the respective antenna coils 14a and 14b are formed in the manner that a single copper wire 13 is wound about each of two ferrite cores 12a and 12b, and respective antenna coils 14a and 14b are thereby formed.
Alternatively, the configuration of the ferrite antenna 11 can be such that the respective antenna coils 14a and 14b are formed in the manner that the copper wire 13 formed by as a conductive pattern on a flexible substrate is wounded on the respective ferrite cores 12a and 12b. Then, respective assemblies of wire windings on the ferrite cores 12a and 12b are connected together by the copper wire.
As shown by a broken line 15 in FIG. 6B, when the current (current or induction current of the transmission signal) flows through the antenna coils 14a and 14b of the ferrite antenna 11, the current passes through the insides of the respective ferrite cores 12a and 12b, and a magnetic flux occurs spreading in exactly identical patterns to one another on both sides of the ferrite cores 12a and 12b. More specifically, the ferrite antenna 11 exhibits the same characteristics of directivity on both sides of the ferrite cores 12a and 12b. 
With reference to FIG. 6C, in order to enhance the directivity of the ferrite antenna 11, a metal plate 16 is disposed close to the ferrite cores 12a and 12b on which the copper wire 13 is wound.
More specifically, when the metal plate 16 is disposed close to the ferrite antenna 11 in the state the magnetic flux occurs as shown in FIG. 6B, overcurrent occurs in the direction of canceling the magnetic flux in the metal plate 16. Then, as shown by a broken line 17 in FIG. 6C, the magnetic flux on the side to which the metal plate 16 is placed close (the side will be referred to as “non-communication direction side”) attenuates.
As the magnetic flux on the non-communication direction side is cancelled, self-inductances of the antenna coils 14a and 14b are reduced, whereby the current flowing through the antenna coils 14a and 14b are increased. Then, as shown by a broken line 18 in FIG. 6C, an increased amount of the current causes an increase in the magnetic flux on the opposite side (communication direction side) to the side to which the metal plate 16 is placed close. Consequently, the directivity on the communication direction side is enhanced.
Notes
Referential publications regarding the related techniques being described:
(Patent Document 1) Japanese Unexamined Patent Application Publication No. 2006-197510
(Patent Document 2) Japanese Unexamined Patent Application Publication No. 2006-031508
(Patent Document 3) Japanese Unexamined Patent Application Publication No. 09-200117